Ecological restoration can improve soil fertility and have a significant impact on the soil nitrogen cycle. Nitrogen (N) is an essential nutrient element for plant growth and development, and also an important factor limiting soil productivity. As an important part of soil nitrogen, the composition and proportion of soil organic nitrogen components can directly or indirectly affect the difficulty of soil organic nitrogen mineralization and nitrogen availability, and then affect soil fertility. However, the current studies on soil nitrogen under ecological restoration mainly focus on nitrogen accumulation and nitrogen mineralization, while there are relatively few studies on changes in soil organic nitrogen components, especially in alpine regions. Therefore, in this study, three restoration pattern of mixed forage (MG), single shrub (SA) and shrub combination (SG) that have been restored continuously for 15 years in northwest Sichuan, China, were taken as the research object, and natural sandy land (CK) without manual intervention was taken as the control. Through field sampling and laboratory analysis, the characteristics of the soil nitrogen content and its proportion to soil total nitrogen (TN) under ecological restoration in alpine sandy land in northwest Sichuan, China, were investigated, and the correlation between the nitrogen content and soil physicochemical properties was analyzed. The results showed that the three ecological restoration patterns significantly increased the contents of acylated ammonium nitrogen (AMMN), acid-lyzed amino sugar nitrogen (ASN), acid-lyzed amino acid nitrogen (AAN), acid-lyzed unknown nitrogen (HUN), acid-lyzed total nitrogen (AHN) and non-acid-lyzed nitrogen (NHN) in soil, and the change trend was consistent with that of soil TN. Ecological restoration improved soil nitrogen mineralization and storage capacity by increasing the proportion of AAN, HUN and NHN to soil TN, and the effect was most obvious in the MG pattern 20–40 cm and SG pattern 40–60 cm soil layers. In general, except ASN, the soil nitrogen content was positively correlated with the soil TN, soil water content (SWC) and soil organic carbon (SOC), and negatively correlated with the soil bulk density (BD) and pH. The results of this study will help us to understand the supply capacity of soil nitrogen under ecological restoration and provide a scientific basis for the selection of an ecological restoration mode and the improvement of the restoration effect and efficiency in alpine sandy land.
